I volunteer at a Missouri Department of Conservation range, and they only require that the guns , and especially, the chambers be empty. Actions are to be locked open, but when that is not possible, they have these empty chamber flags available. They are a good idea for this use, but I could see their use in other situations, also. Beware, though, they will melt if inserted in a hot chamber. I've never seen them melted to the gun, but many were deformed from the hot guns.
Brian